Electro-repressurization



y 1954 E. SARAPUU 3,141,504

ELECTRO-REPRESSURIZATION Filed Jan. 21. 1960 fly. [0

IN V EN TOR.

N %K [r/ch 50/0 u ATTONEY.

United States Patent Filed Jan. 21, 1960, Ser. No. 3,897 1 Claim. (Cl. 166-36) This invention relates to the recovery of oil from oil horizons in either primary or secondary production and refers more particularly to methods for aiding recovery of petoleum employing electrically explosive or detonable gaseous bodies in the oil horizon itself.

Petroleum is usually found associated with sandstone or porous limestone deposits situated between impervious and impermeable layers of shale, rock or the like. In most discovery fields, particularly in deeper horizons or deposits, the oil contains quantities of lighter gaseous hydrocarbons, such as methane, ethane, propane, etc. which may exist as free gases in contact with the oil (gas cap) or dissolved in the oil itself, in either case to greater or lesser degree. When a pressurized oil bearing sand is reached by drilling, oil is conventionally produced by flowing it to the surface under the expansive force of the gases at horizon pressures, whether the gases are free or dissolved in the liquid oil. Wells are conventionally produced by virtue of the pressure within the sand until most of the gas associated with the oil escapes and the motive power bringing the oil to the surface is dissipated.

Pumping may then be initiated in order to continue to recover oil. During the pumping operation, gas associated with the oil continues to escape with the oil produced.

Once the primary recovery becomes uneconomical, the method of re-energizing the reservoir system is commonly practiced. This commonly involves forcing back into selected wells in a field air, gas, water, solvents, etc. which penetrate the sand, thereby forcing further crude oil to the producing wells where it is removed by pumping or flowing. The repressurization media gas or air, etc. escapes with the oil produced and may pick up or entrain more volatile portions of the residual oil, thus removing them from the well. At the end of the repressuring period, and whether or not the field has been water flooded, a large quantity, often more than half, of the oil known to be initially present is left as residual oil in the horizon. At the present time the problem of recovering this residual oil has become increasingly important, due not only to the large consumption of petroleum and petroleum products but also to the lessening number of new discovery fields.

It has heretofore been recognized that oil may be recovered by applying heat to oil-containing sands in earth formations. By means of heating an oil-containing sand, the heavier hydrocarbons which may be clogging the pores of the sand may be rendered less viscous, thereby facilitating the flow thereof through the sand. In addition, such heating tends to distill more volatile hydrocarbons in the sand to aid their movement therethrough.

Various methods have been proposed to effect a heating of an oil sand. Thus, actual combustion of a portion of the residual oil (utilizing air furnished to the sand under pressure or air in combination with a combustible gas to initiate and support combustion) has been utilized. Additionally, passing heated products of combustion in gaseous form through the oil sand in order to eiiect heating thereof has been used.

The art has recognized in specific patents the importance of heating hydrocarbons in earth formations to decrease their visccsity. Thus Patent No. 1,372,743, Gard- 3,1415% Patented July 21, 1964 ice ner, issued March 29, 1921 shows the passing of an electric current between underground electrodes between two well bores (heating the fluid in the well bores). Patent 2,188,737, Hixon, issued January 30, 1940, shows the provision of a subterranean chamber in an oil horizon, passing combustible gas into the chamber and burning it there to permeate throughout the sand. Patent 2,584,605, Merriam et al., issued February 5, 1952, shows initiating and supporting combustion of coal in a coal formation and then passing the heated products of this combustion into an oil horizon in the same earth formation. Patent 2,630,307, Martin issued March 3, 1953, shows producing a cavity or opening in an oil formation by use of an explosive, then flowing combustible gas into said chamber and burning it there to force products of combustion and distillation through the horizon to a withdrawal well. Patent 2,670,047, Mayes et al., issued February 23, 1954, shows, in a process of continuously supporting material into the combustion gas input well in the zone of the oil horizon.

Many disadvantages have proved to be inherent to these methods. Thus, for example, in any direct continuous combustion of a portion of the residual oil, it is difficult to control the extent of combustion, difficult to maintain the underground combustion, and destruction of an appreciable portion of the oil itself is involved. Additionally, combustion in but a limited zone is conventionally proposed, with expensive well completions and accessory apparatus required to initiate combustion, maintain combustion and recover the oil.

Therefore, an object of the instant invention is to provide a new process of treating an oil sand to improve production characteristics by repressurization and heating thereof.

Another object of the invention is to provide a new process for treating a large zone of an oil horizon wherein the internal pressure and temperature of the sand may be increased, the viscosity of the oil in the sands may be lowered and oil production may be improved or enhanced.

Another object of the invention is to provide a process of increasing pressure and temperature in an oil horizon by means of detonating a body of explosive gas mixture in an extensive portion thereof, said process also aiding in controlling the barrier characteristics of waxes and asphaltic ends so as to permit freer oil flow to production well bores.

Another object of the invention is to provide a method of recovering oil from oil horizons employing high temperature repressurization within the oil horizon, a minimum of equipment being required to practice the process and no continuous passage of combustible gas into the oil horizon or maintenance of combustion therein to support the process.

Other and further objects of the invention will appear in the course of the following description thereof.

In the drawings, which form a part of the instant invention and are to be read in conjunction therewith, an embodiment of the invention is shown and, in the various views, like numerals are employed to indicate like parts.

FIG. 1 is a cross-sectional view through an earth formation having an oil horizon positioned therein, the apparatus required to practice the inventive method shown positioned in well bores penetrating the horizon.

FIG. 2 is a side view of a second type of electrode employed in practicing the invention.

Referring first to FIG. 1, therein is shown a typical application of apparatus to practice the inventive method in an oil horizon. The showing is to some extent schematic, but involves actual apparatus employed to practice the invention. At 11) is designated the ground level of the earth formation 11 which overlies oil formation 12 and overburden 13 of nonpermeable rock, shale, etc. which furnish a reservoir for the hydrocarbon containing oil horizon 12.

At 14 is shown a first well bore which is drilled through earth formation 11 and overburden 13 into and optionally through or to the bottom of horizon 12. The criterion of extension of the well bore 14 into sand or horizon 12 is to provide suflicient well bore face within the horizon to permit the flowing of an adequate or desired quantity of gas into the horizon per unit time or to permit access to any particular permeable streak or zone of the horizon for the gas to be flowed thereinto. Such streaks and zones may be identified by conventional core analysis. The upper portion of well bore 14 down to horizon 12 may be of greater diameter than the well bore in the horizon or the same diameter as desired. If the well casing is inserted in well bore 14 before drilling is continued into the sand, the hole into the sand is rat holed within the larger Well bore and casing. Suitable pipe or casing 15 is run from the surface at least to the top of oil horizon 12 and preferably slightly thereinto. If there are any earth formation zones above the oil horizon which need to be sealed to avoid leakage of formation fluids into the well bore, these may be sealed or cemented in conventional manner and, if there is any problem of leakage of horizon fluid into permeable zones above the formation, the casing 15 may be sealed as required to avoid such leakage and fluid loss. Casing 15 extends continuously from horizon 12 to the surface and has input flow line or pipe 16 with valve 17 thereon connected thereto. Suitable air compressor or gas pump of conventional sort is provided at 18 on line 16. Casing 15 is sealed at the top thereof by well head 19 of conventional sort, it being understood that conventional tubing with associated liquid pumping means for raising liquid from the horizon 12 may be inserted in sealing fashion therethrough, if desired. This conventional apparatus is not shown.

To the left and to the right in FIG. 1 are shown combined electrification and production wells. The right-hand well in FIG. 1 will be first described, it being understood that the well completion and apparatus in the left-hand well are identical. Parts identical and common to both well completions will be numbered the same in the lefthand well of FIG. 1, but primed.

Well bore 20 is drilled at least to the top of and optionally through or into horizon 12 to the level desired. If it is desired to insert the casing 21 of well bore 211 in the well before drilling the horizon 12, the well bore into the horizon 12 is rat holed within casing 21 and the greater diameter upper portion of the well bore. Casing 21 is run at least to the top of the oil horizon and preferably slightly thereinto. Casing 21 may be, particularly in the vicinity of horizon 12, of electrically insulating material (such as plastic) or may have an electrically insulating sheath 22 of tape or other material wrapped or formed therearound such as a layer of insulating plastic on the outside thereof. Casing 21 may be sealed or cemented at any portion of its length from the ground surface to the horizon 12 as in the manner of casing 15 to avoid leakage of formation fluids into the well bore or leakage of horizon fluids into the formation. Casing 21 has a well head 23 and optionally has flow line 24 thereon with valve 25 controlling flow therethrough.

A hollow electricity-conducting metal tubing 26 extends downwardly through well head 23 into horizon 12 to a desired depth. Insulator 27 electrically insulates tubing 26 from casing well head 23, while electrically insulating centralizers 28 of any suitable material as plastic or rubber are periodically spaced and positioned along the length of tubing 26 so as to prevent any electricity-conducting contact between the tubing 26 and casing 21. Centralizers 28 must be particularly spaced near the tubing joints, as well. Tubing 26 preferably has an electrically insulating sheath 29 of plastic or insulating tape surrounding its entire length within casing 21, and in any case has such insulation sheath from its lower end in the horizon to a level above any contemplated liquid level in the well bore 20. Flow line 30 with valve 30a controlling flow therethrough having pump 31 of conventional type connected therewith is taken from the top of tubing 26 above well head 23. A form of electrode for applying current in a relatively shallow formation is shown connected to the lower end of tubing 26 and contacting the well bore wall within the horizon at a desired level. Perforated electrode shells 32 are biased apart by spring 32a and connected to tubing 26 by leaf springs 32b. Coil spring 32a may be so constructed as tobe compressed during electrode insertion in the well bore and released thereafter in the horizon.

Electrical connection 33 to the upper end of tubing 26 connects cable or wire conductor 34 of conventional sort to a source of electrical power schematically shown at 35, preferably an alternating current source of suflicient current capacity and voltage level. Suitable generators and transformers of conventional type or connection to an electrical power line or other source of power of the desired magnitude and intensity may be employed.

The entire purpose of the described apparatus and well completion is to provide a path for current flow from the source of power 35 through conductors: 34 and 34', connections 33 and 33' to tubings 26 and 26, where the current is to be passed without arcing to the electrode cori= nections to the horizon. The presence of any shorting liquids or vapors in casings 21 and 21' is nullified as pre viously mentioned, by suitably sheathing and insulating the various conductors and casings along their lengths to avoid any short circuits or arcs. Metal to metal contact between the tubing 26 and casing 21 cannot be permitted. It should also be mentioned that, if there is any problem of arcing or current flow between electrodes 32 and 32 and casing 15, the latter must be suitably insulated or sheathed to avoid such bypassing.

My Patent No. 2,795,279, issued June 11, 1957, for Method for Underground Electro-Linking and Electro- Carbonization of Mineral Fuels discloses a process of passing an electrical current through a fuel strata of sufiicient voltage and amperage to overcome what resistance there may be in order to raise the temperature to the point of decomposition of hydrocarbons. This temperature, above 300 C., forms a coke or ramified fixed carbon channel or zone whereby to complete an electrolinking between electrodes. The process of this issued patent may be contrasted with the instant process to dis tinguish the nature of the latter.

Broadly conceived, the instant invention conceives flowing gas through a well bore such as 14 from the ground surface into the oil horizon 12 at super-horizon pressure in suflicient quantity to establish a substantially continuous body of gas surrounding the well bore 14 in the horizon. This body of gas must be of sufiicient areal extent so as to include the two well bores 20 and 20 within its periphery. The gas, when established as the body in the oil horizon must be of such character in situ as to be detonable or explodable therein upon application of sufficient heat thereto. This detonating force or heat is provided by the passage of current between the two electrodes 32 and 32' which communicate with the oil horizon within the periphery of the gas body. The electrical current must be of such voltage and amperage and maintained for such duration as to detonate substantially the entire gas body in situ within the horizon.

In producing the detonable body of gas in the horizon it is contemplated that one of two alternative approaches may be taken. In the first place, a prepared mixture of gas which is explosive or detonable under the conditions of pressure and temperature within the oil horizon is pumped into well bore 14 to form said body. Thus the detonable mixture is made up before passage of it into the horizon. On the other hand, it is contemplated that suitable gas or gases may be pumped into the horizon or forced thereinto which, when combined with the hydrocarbon constituents of the oil horizon, will produce such a detonable mixture. As has been previously noted, numerous light hydrocarbons such as the paraffins methane, ethane, propane, various unsaturated hydrocarbons, ring compounds, etc., conventionally are present in the oil bodies in oil sands. The composition of a given horizon may be more or less accurately determined by core analysis before flowing gas into the horizon. Suitable gases contemplated particularly include air, oxygen, or mixtures of air and oxygen.

In order to evaluate the extent of the detonation and the timing of the detonation, suitable pressure gauges of conventional type (not shown) may be placed on flow lines 24', 24 and 16 to indicate any change in pressure within the horizon. Potential intensity is raised to the desired level at the electrodes, while the pressure gauges are monitored to determine when the detonation takes place. The detonation is accomplished according to present information and analysis by a series of minute arcs passing between current lines of different potential established between the two electrodes in the horizon. The detonable mixture is interspersed between the sand grains and particles in the horizon and thus the explosion is damped by the horizon. The net effect of the explosion is to raise the total energy in the horizon which, depending upon the particular horizon involved, may involve one or more of pressure, temperature, and viscosity changes. It is contemplated, particularly in the case of the flowing only of the oxidizer or oxygen or air into the formation, that testing of pressure or degree of detonation achieved by varying current flows may be required and additional gas input required until the detonable mixture is achieved. While detonation effects extend somewhat beyond the electrifying well bores 20 and 20', the major effects are produced therebetween. Additionally, it is contemplated to space the gas input and electrification wells in various patterns to cover large areas and zones. However, the preferred and typical arrangement will be with the gas injection well centrally of or spaced between at least two electrification wells, as illustrated. Returning to the electrification process involved, sufficient potential must be provided to provide a system of minute arcs between individual current lines in the horizon provided by the potential field. Building current density to a suflicient level for electro-linkage in the manner of my above patent is strongly avoided by avoiding excessive current concentration at small area electrodes with resultant heating elfects in the vicinity of the electrodes. Another indication of successful repressurization is increased flow of liquid in the production well bores 24) and 20'. This liquid is preferably pumped from tubing 26, although in some instances, particularly in shallow fields, it is contemplated that the increased energy within the oil horizon will permit flowing of the oil from tubings 24 and 24' without additional pumping. This is not preferred as it tends to excessively deplete the energy from the horizon.

The pattern of distribution of the detonable gas body depends-upon the permeability pattern in the horizon and field and the quantity of gas employed. However, the gas must be input at sufficient pressure and in suflicient quantity to provide a substantially continuous gas body communicating with both electrodes 32 and 32. The greater the quantity of gas the more the horizons will be energized. Therefore, it is desirable to fill the horizon in the zone desired to be energized at feasible pressures. The position of the electrodes in the horizon relative to the position of the gas body is not too critical as the lines of current flow tend to spread throughout the horizon. However, the criterion is the passage of suflicient current through the gas body to provide the detonation. Thus, by providing a combustible mixture within the horizon in a certain zone having a relatively low ignition temperature and providing suflicient current that the potential difference between current lines is suificient to spark or set olf a series of arcs between sand grains, a damped explosion is set off within the horizon.

While in some horizons, particularly those with either a pronounced segregation between gas and oil, Water and oil or gas, water and oil, it may be desirable to seal off gas caps or aquifer zones of the horizon from the production areas in well bores 20 and 20', in the absence of such excessive demarcation of fluids in the horizon, no particular sealing or isolation of zones of the horizon is required.

It is very important to prevent current leakage except at the electrodes. In order to secure this, the various insulating means, including insulation of casings 21 and 21, tubings 26 and 26', casing 15, etc. have been provided. Any fluids between the electrification tubings and casings in the electrification Wells must be considered in avoiding of short circuiting or arcing. The insulation in each case must be of a thickness and effectiveness required to handle the given current involved. Thus, the insulation of the outer casing 21 may be in the form of silicone plastics, insulating tape or, in some cases, the use of plastic or fiberglass casing or pipe.

It is also contemplated to repeatedly detonate within the same horizon one or more extra charges of detonable gas, in so far as it is desired to raise the total energy available in the particular zone being treated. Thus, after a first explosion or detonation, if the pressure temperature or flow effects are not as desired, a second charge of detonable gas or suitable gas to mix with the hydrocarbons to provide the detonable mixture in the well is flowed into the horizon in sufiicient quantity to again communicate with the electrification wells. Again the current flow is built up until a second detonation is achieved. This process may be repeated over and over as required to produce the desired effects in the well bore. In such cases, it is contemplated that the required gas mixtures may change depending upon the changes in the horizon. Additionally, it is contemplated that the current requirements may vary as the characteristics of the horizon are effected.

A very important procedure contemplates the flowing of an inert, noncombustible gas into the pressurization casing 15 after the initial charge is placed in the formation in sufficient quantity and under sufficient super-horizon pressure to drive all the detonable gas into the formation. Additionally, it is contemplated that the cap of the gasification casing at 19 be designed to rupture entirely across its entire area at a fixed pressure or when such pressure is exceeded, Additionally, it is also contemplated flowing sufficient inert gas such as nitrogen into the casings 21 and 21' to maintain these casings free of detonable gas mixture. Such maintenance of the casing volumes and well bore volumes free of detonable gas with the detonable mixture only in the horizon aids in protection of the Well completions and maintenance of the detonation in the sand itself, where it is desired. Additionally, pressure relief means of various conventional types may be provided in casings 21 and 21' for extra protection to these well completions, if desired.

As a specfic example of a detonable gas mixture which may be employed, applicant discloses methane. The mixture of methane with air or oxygen forms a typical detonable mixture. Other hydrocarbon gas mixtures which may be employed or created in an oil horizon include mixtures of air or oxygen with ethane, propane, butane and natural gas. The following table from the US. Bureau of Mines Bulletin No. 279, 1939, Coward & Jones Limits of Inflammability of Gases and Vapors gives the explosion limits to these particular gases and also gasoline vapor at normal temperature and pressure in air.

Vol. Percent Methane 5.2414.02 Ethane 3.2212.45 Propane 2.37- 9.50 Butane 1.86- 8.41 Natural Gas 4.85-13.75 Gasoline 1.40- 6.50

Referring to FIG. 2, therein is shown a second type of electrode employed in a thicker formation than that shown in the other figure. Thus this comprises a point electrode rather than a split pipe electrode. Tubing 39 is connected by joint 40 to electrode tubing 41. A plurality of rectangular plates 36 are connected to tubing 41 by hollow conduits 37. Alternatively, coil springs 38 may be connected to the plates 36 and received in the conduits 37 whereby to bias the plates in resilient fashion toward the well bore wall. I In the example shown, the electrode plates contact the well bore wall within a single 180 are.

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth together with other advantages which are obvious and which are inherent to the process.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim:

A process for improving the oil production characteristics of an oil horizon in an earth formation comprising flowing a quantity of gas through a well bore from the ground surface into said horizon at super-horizon pressure in sufiicient quantity to establish a substantially continuous body of gas surrounding said well bore in said horizon, said gas flowed into said horizon of such composition as to form with at least one hydrocarbon in said horizon a detonable gas mixture in said gas body, flowing an inert, nondetonable gas into said wellbore at super-horizon pressure in sufiicient quantity to force all detonable gas in the wellbore into said horizon, and flowing electrical current into said oil horizon within the limits of said gas body after establishment of same in said oil horizon of such character and in such quantity as to detonate said gas body in said horizon.

References Cited in the file of this patent UNITED STATES PATENTS 1,248,689 McAvoy Dec. 4, 1917 2,353,484 Merten et al. July 11, 1944 2,795,279 Sarapuu June 11, 1957 2,818,118 Dixon Dec. 31,1957 2,858,891 Moll et al. Nov. 4, 1958 

